A cryogenic fluid, typically LPG or LNG, has a very low temperature at ambient pressure when stored as a liquid. If this liquid is spilled at or in a vessel all equipment in the vicinity of the spilled liquid will be cooled down to the boiling temperature of the liquid, which for LPG can be typically −50 degrees C. and for LNG typically −164 degrees C. Most vessels and equipment are built of carbon steel that will turn brittle and loose its structural strength at cryogenic temperatures.
Almost all present transfer of LNG from shore to ship or ship to shore or between ships is performed by means of chicksan type loading arms. The arms are located on the quay and the receiving/transfer piping on the manifold located midship on the LNG transportation vessel. The arms and the piping are located outdoor and any spillage will be collected in drip pans underneath the connection point. The main purpose of the drip pans is to collect any liquid spillage and direct it to special storage tanks and to prevent cold liquid to fall onto the steel structure of the vessel. A rapid cool down of the vessels steel structure will cause the steel to be brittle leading to mechanical break-down either due to thermal stress (contraction of part of the part due to low temperature) or mechanical stress (due to reduced strength of the material at low temperature). Any boil off will evaporate to the atmosphere or into a specific system for boil off.
In future transfer systems LNG may be carried through closed spaces inside the ship. This could be for instance in the OCL (Offshore Cryogenic Loading) LNG transfer system, where LNG is received in a purpose built structure in the bow of the ship, in compartments used to transfer LNG to the propulsion or generator motor or in a swivel compartment for instance described in WO 99/38762 or WO 01/34460. There may be further use of transfer of cryogenic fluids from one floating vessel to another floating vessel, by for instance a submerged buoy system, which gives other demands for the transfer system.
The cryogenic liquid is maintained at or close to its boiling point at low temperature and any contact with other material at a higher temperature will result in transfer of heat from the material to the liquid and boil-off of liquid and cool down of the material. The liquid is therefore normally stored in well insulated pipes and tanks and all transfer is carried out with dedicated equipment and according to pre-determined and approved procedures to reduce the risk of spill and accidents.
Another issue in relation to the transfer of cryogenic fluid is that any human exposure to the liquid or cold gas may result in serious injury or death. Protection of personnel can be achieved by either prohibiting personnel to enter the area when cold media is present and can leak out, or by use of adequate protective equipment. At this time protective equipment is limited to space suits and similar clothing which by nature is either prohibitive expensive or not available at such sites. As a general rule unprotected personnel should therefore not be admitted to enclosed spaces where there is a possibility that LNG may escape (from for instance rotating equipment or flanged connections). Equipment inside such space must therefore be remotely operated and monitored and all containers with cryogenic liquid must be drained and inerted before personnel can enter.
Cryogenic liquids are normally not corrosive or destructive to the environment other than by cooling down all exposed material to low temperature and replacing the atmosphere by the product liquid and gas. Cryogenic liquids are normally of a low specific gravity (typically less than 0.5 kg/l) and will float on top of most other liquids. Loose material will tend to sink in the liquid. The boil-off gas will at ambient temperature (+10 degrees C. to say 50 degrees C.) normally be lighter than air and will therefore tend to move upwards and mix with air. However, Boil-off gas at very low temperature will be heavier than air at ambient temperature. A release of cryogenic fluid inside an enclosed compartment will therefore tend to form cold product gas at the bottom and expel the original atmosphere at the top. As the product gas heat up it will tend to rise upwards in the compartment. Removal of product gas should therefore be performed by blowing in air (or inert gas) at as high temperature as practical to evaporate liquid spill and to heat up and expel the cold gas.
As long as liquid cryogenic fluid is present it will exist at its boiling temperature at the actual ambient storage pressure. Any input of heat will result in boil-off and if no heat is added the temperature of all exposed material will fall until the boiling temperature of the liquid has been reached, where after the boil off will be reduced to equalize the heat influx. Equipment inside a compartment can be protected from the low temperature by insulation or by adding heat to the equipment, typically by heating coils or resistance heating elements wrapped around the sensitive parts. Insulation of free standing equipment inside the enclosed space will have a practical limitation in protection time if no heat is added, as it is not possible to insulate any equipment perfectly. With time the equipment inside the insulation will be cooled down to the boiling temperature of the liquid on the outside.
Parts that are insulated on the outside and where heat is added on the inside can be maintained at any desired temperature for any period of time as long as the flow of heat supplied is higher than the heat removed on the exposed side of the insulation. Transfer of heat from one media to another is either done by radiation through vacuum or a gas or by heat flow through the material. Vacuum is the best insulation and reflective material is used together with vacuum, or almost vacuum, to reflect heat radiation. Most insulation principles are therefore based either on vacuum, reflective material or fixed foam, or gas which is a better heat insulator than solid material.
All these measures do not give a satisfactory system for transferal of cryogenic fluid between two units.